Shadow spells can create real effects
Despite being illusions, spells with the [shadow] subtype can create real effects, as described under Magic:
Shadow: A shadow spell creates something that is partially real from extradimensional energy. Such illusions can have real effects. Damage dealt by a shadow illusion is real.
So, for all purposes, regardless if it's a 100% or 20% chance of affecting the target's, if the target fails their saving throw and also fail to resist the shadow effect (the d% percentile roll), the target is affected as if affected by a real copy of that spell.
So, for a shadow illusion of Create Pit, they would fall into an actual extradimentional pit and take damage from it, and unlike figments and phantasms, there is no translucent outline, the effect is real and the means of saving the target, climbing out or even flying out of it work the same as if they suffered from the real spell.
Note that, if the targets resist the effect, they are considered "nonbelievers" and then you roll the d% percentile dice to see if they are still affected by the spell (because the effect is partially real). Those who fail their initial save are automatically affected. So, if the spell creates a 100% copy of the spell, then there is no chance for any creature to become immune to the illusion and its effects, they resist the "illusion" but then a real effect shows up, kind of making it pointless to even have a Will check, as the creature still have to make a save against the copied effect:
Regardless of the result of the save to disbelieve, an affected creature is also allowed any save that the spell being simulated allows, but the save DC is set according to shadow conjuration‘s level (4th) rather than the spell’s normal level.
So, a 100% Shadow Conjuration of Create Pit will create a 100% real extradimentional pit, forcing all creatures in the area to make a Reflex saving throw as a 4th-level version of the spell, either falling into the pit if they fail their Reflex check, or moving out of it if they pass their Reflex check.